Platelet-derived growth factor (PDGF) has been hypothesized to play important roles in driving migration and proliferation of vascular cells during development and in vascular diseases. Attempts to test these hypotheses in vivo have produced results which are incomplete or inconsistent. This motivated us to develop experimental approaches which allow us to directly compare the behavior, in vivo, of cells which can, or cannot, express components of the PDGF/PDGF receptor system. These approaches allow us to determine the roles of the individual cells. At the same time, the approach provides information about the processes of cell migration and clonal expansion that contribute to the assembly of vessels during development, and that may later be involved in vascular changes in vascular diseases. These include processes which contribute to the apparent monoclonality of atherosclerotic lesions and the possible contribution of circulating precursors to the endothelium of vessels during tissue remodeling.